1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to a hydraulic actuator assembly driving the die closing unit of an injection molding machine of the type in which the injection molding die is pushed closed and the closing pressure is produced by a single axially aligned power cylinder.
2. Description of the Prior Art
Various push-type die closing units with axially aligned power cylinders are known from the prior art. Also known is an arrangement of a power cylinder which has a power piston with bypass passages for rapid opening and closing movements of the die closing unit during which the power piston moves through the hydraulic fluid in the power cylinder, rather than displacing fluid from one axial side, while replacement fluid enters the cylinder from the other axial side.
While the bypassable power piston is thus "neutralized" during the rapid opening and closing movements of the die closing unit, its piston rod may be arranged to act as a differential piston for one of the two travel movements. A single-acting auxiliary cylinder extending in the same axis as the power cylinder will then provide the opposite travel movement.
Die closing units with a hydraulic actuator assembly of this type are disclosed in U.S. Pat. No. 4,047,871 and in U.S. Pat. No. 4,105,390. This hydraulic actuator assembly has a piston rod which extends through the power cylinder and into an attached auxiliary cylinder, or travel cylinder, carrying an auxiliary piston at its distal extremity. The power piston has a number of internal axial bypass channels which are surrounded by a circular valve seat. Cooperating with the latter is an axially movable annular valve plunger which, under the influence of a pressure space arranged between the valve plunger and a piston rod shoulder, can be pressed against the valve seat to close the bypass channels, when the power piston is to be pressurized for the creation of an elevated die closing pressure.
The piston rod of the power piston extends forwardly from the power cylinder, into engagement with a movable die carrier frame. The piston rod portion which extends rearwardly into the auxiliary cylinder is of smaller diameter. Accordingly, the piston rod itself is a differential piston, producing a die closing movement, when the power piston bypass is open and the power cylinder is pressurized. The coaxial travel cylinder produces the die closing movement.
This configuration of a hydraulic actuator assembly has the disadvantage of requiring considerable space in the axial direction. Both the power cylinder and the coaxial closing travel cylinder must accommodate a longitudinal stroke which corresponds to the maximum opening movement of the die closing unit, with the result that the axial length of the die closing unit is more than three times the maximum distance of the opening stroke of the die closing unit.
It is also known from the prior art to arrange a multicylinder hydraulic actuator assembly in such a way that smaller drive cylinders extend parallel to, and on diametrally opposite sides of a main cylinder, all three cylinders having their piston rods connected to a transverse plate member to produce a cumulative die closing pressure. Such an arrangement is disclosed in the German Offenlegungsschrift (Published Application) No. 1 529 937.
This publication suggests a hydraulic actuator assembly which has a conventional double-acting main cylinder arranged in the axis of the die closing unit and two smaller clamping cylinders arranged parallel and on diametrally opposite sides of the main cylinder. The die opening and closing movements are produced by the main cylinder, and the two clamping cylinders enter into action only, when the injection molding die is to be clamped shut under high pressure.
This is accomplished by means of clampable pistons in the clamping cylinders which, when subjected to an elevated hydraulic pressure, develop a clamping action between the pistons and their otherwise freely movable piston rods, thereby adding the forces created by the pressure spaces of the clamping cylinders to the force of the main cylinder. A solenoid valve controls two hydraulic lines which connect the high pressure space of the main cylinder with the pressure space of the clamping cylinders.
This arrangement, aside from its complexity in terms of the hydraulically clampable pistons in the lateral clamping cylinders, utilizes the main piston to produce the opening and closing travel of the die closing unit, thus requiring the displacement of large quantities of hydraulic fluid during each machine cycle, if the main cylinder is to be large enough to produce the necessary die closing pressure.